The invention concerns a device for measuring the gradient of a component of a magnetic field (excitation H or induction B), a process for measuring said gradient, and a process for producing said device. Such a device can be produced in integrated form.
This invention applies in particular to measuring electrical current in filiform conductors.
Known from document WO 03044541 is a method for measuring a current in a conductor, implementing the measurement of n-th order derivatives of the magnetic field. This method makes it possible in particular to reduce the disruptions caused by parasitic conductors.
Let xyz be an orthonormal reference, shown in FIG. 1, centered at the point M where one wishes to conduct the measurement, and where y is parallel to the axis of the conductor 1 (and situated in plane yMz). The z axis is colinear to the radius perpendicular to the conductor passing through M and x is tangent to the circle C passing through M whereof the axis is the conductor. The field H created by the current i circulating in the conductor 1 is then oriented along the axis x and the gradient of Hx is oriented on the axis z. Measuring the gradient of the field Hx created by the current i (which drops with 1/r2 as a function of the distance r from the conductor 1), or higher-order derivatives (which drop even more quickly as a function of r), makes it possible to go back up to the current i. This technique makes it possible to better do away, in a noisy environment, for example an electric cabinet, with external disruptions, which allows direct measurement of the field Hx (which is dropping, but only with 1/r). Measuring a current i1 in a conductor C1, in the presence of (disrupting) conductors C2, C3, . . . Cn, which are also passed through by currents i2, i3, . . . in, is thus made easier, due to the measurement of the gradient of the component Hx of the field H (or a derivative of an order higher than 1 of the same component) created by said conductor, at a short distance therefrom, than direct measurement of the field Hx. A configuration is thus sought in which the contributions to the measured size, caused by the disrupting conductors, are extremely low, while that caused by the main conductor C1 is as strong as possible.
The implementation of this technique poses several problems.
To maximize the signal, it is advantageous to position the sensors (N sensors for an N−1 order derivative) very close to the conductors where one wishes to perform the measurement. The problem then arises of reducing the distances separating these sensors from the conductor 1, as well as the distances separating the sensors from each other, as much as possible.
Thus, to effectively and precisely measure the current, it is advisable to perform the measurement at points situated as close as possible to the conductor 1, at distance r from the center thereof. It would also be advisable, to precisely measure the derivatives of the field, for the distances between sensors to be smaller than r and, if possible, small in view of r.
One solution would be to miniaturize the sensors. But if the sensors are miniaturized, their environment (and in particular their packaging) could impose minimal dimensional constraints, which can sometimes be bothersome, as in the configuration of FIG. 5 described in document U.S. Pat. No. 6,154,023.
It is therefore difficult, in certain cases, to reduce the distance d between the different sensors.
It would also be advantageous to be able to use, for this type of measurement, very directional sensors, such as micro fluxgates, or magneto-impedances, or magneto-resistances with flux guides. Yet the magnetic circuits of these sensors disrupt each other when the sensors are brought too close, resulting in a less precise measurement.
Precise measurement of the gradient also demands sensors whereof the positions in relation to each other are very well known. Moreover, if they are very directional (which is advantageous to perform a non-noisy measurement), they must be parallel to each other, which is very delicate to achieve with discrete sensors, in particular when they are miniaturized.
Another problem arises during use of a gradient sensor in an industrial environment. In this context it may sometimes be necessary to use the sensor inside a magnetic shielding, which, although it has the advantage of reducing disrupting fields, also has the drawback of creating a relatively significant field gradient in its immediate vicinity. The problem then arises of having a gradient sensor small enough to avoid one of the elementary sensors making it up being too close to the shielding, which could disrupt the measurement.
The problem also arises of being able to have a sensor structure that can be made in collective technology, for example with machines of the types used in microelectronics or micro- or nano-technologies.